The present invention relates generally to a zoom lens system, and more particularly to a two-unit zoom lens system that comprises six groups, each consisting of one lens, and has a zoom ratio of about 2. This two-unit zoom lens system is best suited for lens shutter cameras having an unlimited back focus length, and so on.
So far, two-unit or positive-negative, or three-unit or positive-positive-negative and negative-positive-negative types of zoom lenses have been known as typical zoom lenses for lens shutter cameras.
The "two-unit" mentioned above means the minimum unit number that can achieve zooming, and so many approaches have now been proposed of two-unit zoom lenses, because of their possibilities of making lens barrels and driving mechanisms simple and reducing the number of the lenses involved.
However, the two-unit type of zoom lens has been considered to be unsuitable for attaining high zoom ratios, partly because the amount of movement of each unit is likely to increase in association of zooming and partly because aberration variations incidental to zooming are large (this is particularly true of the curvature of field at an intermediate focal length).
The three-unit type of zoom lens, on the other hand, lends itself well fit for attaining high zoom ratios, partly because each unit can share a zoom ratio to reduce its amount of movement in association with zooming and partly because aberration variations can be well corrected. Because of comprising three lens units, however, they have some demerits, for instance, their lens barrel structures and driving mechanisms being likely to become complicated, an increased number of lenses, and so on.
As will be referred to later, an object of the invention is to obtain an inexpensive zoom lens system that comprises a reduced number of lenses and makes use of plastic material. In other words, it has now been found that a purposive lens system, if it is allowed to have a zoom ratio of at most about 2, is achievable by taking advantage of the "two-unit" type.
Until now such a two-unit type of zoom lenses comprising about six lenses have been known from JP-A-62-138818, 63-266413, 63-311224, 2-73322, 2-120714, 3-116110, and so on.
Of these, the zoom lenses set forth in JP-A-62-138818, 63-266413 and 63-311224 use aspherical glass lenses for aberration correction, and are of good-enough performance. However, the use of aspherical glass lenses is still unfavorable in terms of cost, in spite of recent considerable progresses made in technology.
The zoom lenses shown in JP-A-2-73322 and 2-120714 are all constructed from polished glass lenses, and so are improved in terms of cost over the first-mentioned three zoom lens systems, but are less than satisfactory in terms of spherical aberration and coma correction.
The zoom lens system referred to in JP-A-3-116110 comprises five lens groups or six lenses, and utilizes plastic lenses for cost reduction. However, the use of a positive lens, a negative lens and a cemented positive lens of positive and negative powers renders spherical aberration and coma correction insufficient. In addition, its focal length at the telephoto end is as short as 60 mm.